Recently, with practical implementation of a blue light-emitting diode (LED), development of a white LED utilizing the blue LED is being aggressively sought. The white LED ensures low power consumption and extended life compared with existing white light sources, and therefore its application to liquid crystal panel backlight, indoor or outdoor lighting device, and the like is expanding.
The white LED developed at present is obtained by coating a Ce-doped YAG (yttrium.aluminum.garnet) on a surface of a blue LED. However, the Ce-doped YAG has a fluorescence wavelength in the vicinity of 530 nm and when the color of this fluorescence and the light of a blue LED are mixed to provide white light, blue-tinted white light results and good white light cannot be obtained.
On the other hand, an α-sialon-based phosphor activated by a rare earth element is known to emit fluorescence with a longer wavelength than the fluorescence wavelength of Ce-doped YAG (see Japanese Unexamined Patent Publication (Kokai) No. 2002-363554). When a white LED is fabricated using fluorescence of such sialon, a white LED giving a bulb color at a lower color temperature than a white LED using YAG can be produced.
Also, in J. Phys. Chem., B2004, 108, 12027-12031, a sialon-based phosphor having a compositional formula represented by MxSi12−(m+n)Alm+nOnN16−n gives a maximum intensity at m=2.8, and a peak wavelength in the vicinity of 595 nm is obtained there. This fluorescence wavelength is suitable for a white LED with a low color temperature as in a bulb color, but a white LED with a high color temperature, such as daytime white color or daylight color higher in the color temperature, cannot be produced.
The daytime white color and daylight color have a wide range of applications including not only lighting but also backlight of a liquid crystal display device, etc., and their need is greater than that for bulb color. To meet this need, fluorescence with a shorter wavelength is required of the sialon-based phosphor. However, as understood from J. Phys. Chem., B2004, 108, 12027-12031, a Ca-containing α sialon phosphor is reduced in the fluorescence intensity when the fluorescence wavelength is shifted to the shorter wavelength side than 595 nm. Accordingly, it has been difficult to produce a sialon-based phosphor capable of emitting fluorescence at a short wavelength suitable for producing a high-luminance LED of daytime white color or daylight color by combining the phosphor with a blue LED.
To solve this problem, WO 2007/004493 A1 discloses a Li (lithium)-containing α-sialon-based phosphor. This sialon can emit fluorescence at a short wavelength compared with the Ca-containing α-sialon-based phosphor. However, in the disclosure above, the Li-containing α-sialon-based phosphor is obtained in an atmosphere under nitrogen pressure of 1 MPa and in view of a cumbersome production process or use of a production apparatus capable of withstanding a high-temperature high-pressure nitrogen gas, the phosphor is costly produce to. Also, x1 indicating the Li content in the above-described compositional formula of sialon is an abnormally large value of 1.2≦x1≦2.4, and a Li-containing α-sialon-based phosphor having a desired composition is difficult to produce with good reproducibility.
As for the report on a Li-containing α-sialon-based phosphor, Japanese Unexamined Patent Publication (Kokai) No. 2004-67837 is known in addition to WO '493, but the Li-containing α-sialon-based phosphor disclosed has a fluorescence wavelength of 585 nm and differs in the composition from the Li-containing α-sialon-based phosphor. With such a fluorescence wavelength, even when combined with a blue light-emitting diode, an LED of daytime white color or daylight color cannot be obtained.
Furthermore, in WO '493, the form or aggregation state of particles is not considered. In WO '493, a Li-containing α-sialon-based phosphor is produced using crystalline silicon nitride. In the case of a Ca-containing α-sialon-based phosphor, when crystalline silicon nitride is used, this forms a secondary particle where small primary particles are strongly aggregated (fused). Such a case is seen in FIGS. 1 to 8 of Japanese Unexamined Patent Publication (Kokai) No. 2006-152069. It is presumed that the same occurs in the case of a Li-containing α-sialon-based phosphor.
The form or aggregation state of particles of a phosphor powder affects light scattering, absorption and in turn, fluorescence intensity and furthermore, also affects the slurry properties when coating the phosphor. The slurry properties are an important factor in the production process.
The effect on the fluorescence intensity is described below. In a phosphor, irrespective of the size of a primary particles or secondary particles, when the particle size is reduced to about a submicron, light scattering is increased to lower the absorption and the fluorescence intensity is decreased. To avoid this, a method of producing large secondary particles by aggregating submicron primary particles may be considered. However, when such powder is pulverized or is subjected to various handlings, the submicron primary particles fall off, as a result, it is difficult to avoid the effect of the primary particles. Also, in the case of a secondary particle formed by aggregating small primary particles, fine irregularities are produced on the secondary particle surface, and this is considered to result in light scattering and a decrease in the fluorescence intensity.
In addition, when the aggregation to a secondary particle is firm, strong pulverization is required and incorporation of impurities from the pulverizer occurs. If a component participating in light absorption is immixed even in a small amount, the characteristics of the phosphor are greatly deteriorated, and therefore strong pulverization is not preferred.
Furthermore, when the particle size grows to tens of micron or more, this gives rise to color unevenness or the like at the time of fabricating a product such as white LED, and products with stable quality cannot be fabricated. On the other hand, to obtain a high-quality phosphor, i.e., a phosphor having high fluorescence intensity, a particle with high crystallinity is necessary. From this viewpoint, the primary particle is preferably a large crystal. The reason therefor is that when the crystal size is small, the fluorescence intensity decreases due to surface defects.
Considering these conditions, a phosphor with good characteristics is preferably a powder in which primary particles are distributed in the range of 1 to 20 μM without aggregation and the powder is composed of particles having a larger size in this particle size range.
In Japanese Unexamined Patent Publication (Kokai) Nos. 2002-363554 and 2006-321921, the primary particle size of Ca-containing α-sialon phosphor is already known, but studies on the primary particle of Li-containing α-sialon are not sufficiently made in JP '337 and JP '921. Despite the disclosure of JP '337 and JP '921, the growth of a primary particle of Li-containing α-sialon cannot be estimated to be the same as that of Ca-containing α-sialon, because Li is an easily evaporable element or the substance related to Li may form a compound having a relatively low melting point.
As regards the phosphor, a technique of using a flux is widely employed as a technique for regulating the form of primary particles. For growing a large primary particle, using a flux may also be considered in the Li-containing α-sialon. In WO '493, fluoride, chloride, iodide, bromide and phosphate of Li, Na, K, Mg, Ca, Sr, Ba, Al and Eu, particularly, lithium fluoride, calcium fluoride and aluminum fluoride, are pointed out as flux, but their effects are not specifically described, and the technique disclosed merely suggests a general technique.
It could therefore be helpful to provide a phosphor having high fluorescence intensity and emitting a fluorescence color making it possible to produce a white light-emitting diode of daytime white color or daylight color by combining the phosphor with a blue LED.
It could also be helpful to provide a Li-containing α-sialon phosphor powder having high fluorescence intensity and having excellent properties as a phosphor powder by controlling the primary particle of Li-containing α-sialon. Such a Li-containing α-sialon can produce a high-efficiency white light-emitting diode of daytime white color or daylight color by combining it with an ultraviolet-to-blue LED.
It could further be helpful to provide a lighting device such as white LED of daytime white color or daylight color by providing a Li-containing α-sialon-based phosphor having high fluorescence intensity and using an ultraviolet or blue LED as a light source.
It could yet further be helpful to achieve high luminance and stable color tone of an image evaluation device having an excitation source such as electron beam.
It could also be helpful to provide a novel production process where a sialon-based phosphor capable of emitting the above-described fluorescence color with high intensity can be obtained in a high yield.